Preregulation of high voltage supply circuit for a medical nuclear spectrometer

ABSTRACT

A modular, all transistorized, direct-coupled, pulse-height spectrometer for use in nuclear medical measurements is provided with means for the specific control of noise level, overloads, linearity and voltage regulation, together with compactness. These features make the present spectrometer substantially superior to the prior art tube-type spectrometers.

United States Patent Inventors Marion M. Satterfield;

George R. Dyer, Oak Ridge, Tenn.

Mar. 7, 1969 Mar. 9, 1971 the United States of America as represented by the United States Atomic Energy Commission Appl. No. Filed Patented Assignee PREREGULATION OF HIGH VOLTAGE SUPPLY CIRCUIT FOR A MEDICAL NUCLEAR SPECTROMETER 4 Claims, 12 Drawing Figs.

U.S. Cl 250/71.5, 250/83.3, 250/105, 321/18, 323/22 Int. Cl. G0lt1/17 Field of Search 250/71 .5,

[56] References Cited UNITED STATES PATENTS 3,320,419 5/1967 Thomas et a1. 250/7l.5 3,375,428 3/1968 Mitchell 323/22SCR Primary Examiner-James W. Lawrence Assistant Examiner-M0rton J. Frome Attorney-Roland A. Anderson HIGH VOLTAGE SUPPLY r l TRANSFORMER PASS IL ELEMENT RECTIFIER I FILTER I DETECTOR I I T ,1

L J PREAMPLIFIER i T T 2 CHARGE I szusmve DIFF. STAGE L... .J

LINEAR AMPLIFIER 3 I SHAPING GAIN SECOND I AMPLIFIER STAGE DIFF. I

PULSE HEIGHT ANALYZER l I I |(4 ANTI-COINCIDENCE cmcun i LOWER msc I L.- l

SCALER-TIMER HOWLER MIXER AND OUTPUT OSCILLATOR ZOO-25O kc PATENIEIIIIAR a I97| SHEET 1 IIF 7 HIGH VOLTAGE SUPPLY TRANSFORMER RECTIFIER FILTER PASS ELEMENT I REFERENCE AMPLIFIER PREAMPLIFIER FIRST DIFF.

CHARGE SENSITIVE STAGE I I l 1..

DETECTOR SECOND DIFF.

ANTI-COINCIDENCE CIRCUIT GAIN STAGE LINEAR AMPLIFIE PULSE HEIGHT ANALYZER UPPER DISC SHAPING AMPLIFIER HOWLER OSCILLATOR MIXER AND OUTPUT INTEGRATOR I I I SCALER-TIMER OSCILLATOR 200- 25 O kc GATE PATENTEUHAR man BQSBQJUEL SHEET 2 [IF 7 BY George R. Dyer WM /m ATTORNEY.

Lu Ln LI". INVENTORS, J Marlon M. SafferfIe/d PATENTEUHAR 9m! sum" 3 OF 7 INVENTOI'LS. Mar/0n M. SaHerfIeId BY George R. Dyer flwww ATTORNEY.

PATENTEU MAR. 91971 SHEET R 0F 7 All INVENTORS.

Marion M. SaHerfie/d By George R. Dyer ATTORNEY PATENTEDHAR SIB?! 3569.701

sum 5 OF 7 RESET FOR 87,88

RESET T'MER LAM P RESET DECADE RE SET IN PUT INVENTORS.

Marion M. Sufferfield COUNTS l .5 2- BY eorge R. Dyer J wagq/ ATTORNEY.

PATENTEU MAR 91971 SHEET 6 0F 7 lib NT INVENTORS. Marion M. SaHerfie/d BY George R. Dyer W 4- W I ATTORNEY.

PATENTED MAR 9197i 3569701 SHEET 7 BF 7 IN VENTORS,

Marion M. Sarierfield BY George R. Dyer m ATTORNEY.

i PREREGULATTON OF HIGH VOLTAGE SUPPLY I CTRCUET FOR A MEDECAL NUCLEAR SPECTROMETER I BACKGROUND OF THE INVENTION it has been the practice in certain diagnostic examinations and in biological treatments, particularly of humans, to inject into or feed the patient a substance containing an isotope which is a gamma ray emitter. Usually the substance is so chosen that it has a special affinity for the organ to be examined or treated, for example, radioactive iodine which has an affinity for the thyroid. It is also desirable to limit the dosage of radioactive material in the human body to minimize radiation damage to healthy tissue.

The location concentration of the substances must be ascertained in order to determine the uptake and distribution in the tissue being analyzed. One system for scintillation counting of such substances-radioiodine, for example, has been described in U.S. Pat. No. 2,942,109 issued Jun. 21, 1960, and having a common assignee with the present application. The present invention is an improvement over the above patent. The spectrometer described in the above patent is a vacuum tube device and was designed to meet a specific need by the use of a specially designed collimator. However, the above prior system is not as compact as desired, it is not direct-coupled throughout and is thus subject to baseline shifts with changes in count-rate and with overloads, and there is no provision therein for preregulating the AC supply voltage .which becomes desirable when transistors are used in the high voltage power supply as is done in the present invention, since extreme variations in line voltage could destroy the power supply if operated without the preregulating feature to be described below. The present invention was conceived to overcome the limitations of the prior system as set forth above.

SUMMARY OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

lt isthe object of the present invention to provide an improved medical nuclear spectrometer wherein the specific control of noise level, overloads, linearity and voltage regulation can be achieved while at the same time providing a compact unit.

The above object has been accomplished in the present invention by providing an all-transistorized, direct-coupled,

pulse-height spectrometer including means for preregulating the high-voltage power supply in a manner to be described below. Thus, by using only transistors in the present system, a very compact unit is possible; by employing substantially complete direct coupling within and between the various components of the system, the linearity of the system is substantially improved; and by providing means for preregulating the high voltage power supply, a substantially constant output voltage therefrom is ensured to prevent overloading of the transistors used therein regardless of any extreme variations in input line voltage. The present invention provides for a spectrometer that meets the above object and that will operate in a more efficient and accurate manner than is possible with the above-mentioned prior system.

BRlEF DESCRIPTION OF THE DRAWINGS Other advantages of the present invention will be apparent from the following description and the drawings in which:

E16. 1 is a block diagram of the overall system of the present invention;

FlG. 2 is a schematic circuit diagram of the preamplifier portion of the invention;

FIG. 3a, 3b is a schematic circuit diagram of the linear arnplifier portion of the invention;

H6. 40, 4b is a schematic circuit diagram of the pulseheight analyzer portion of the invention;

F16. 5a, 5b is a logic and signal flow diagram of the scalertimer portion of the invention;

FIG. 6a, b is a schematic circuit diagram of an auditory counting rate indicator for the invention; and

H6. in, 7b is a schematic circuit diagram of the high voltage supply for the invention.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT The present spectrometer may be divided into six basic modular sections as illustrated in the block diagram of FIG. 1. These modules are: the preamplifier circuit 2; the linear amplifier 3; the signal pulse-height analyzer 4; the scaler-timer 5; the auditory count-rate indicator 6; and the high voltage supply 7. Each of these sections of the improved design will be described separately hereinbelow.

PREAMPLIFIER The preamplifier module 2 of the present invention is shown in FIG. 2. The photomultiplier tube 1 (to the left), for receiving light signals from a sodium iodide detector (not shown), is included as this preamplifier is physically located at the PM tube base. The preamplifier is located in the detector assembly for two reasons: to eliminate the noise otherwise generated at the input of the charge-sensitive amplifier by the capacitance of the connecting cable; and to permit driving of the cable at low impedance with a narrower pulse as a result of the first differentiation within the preamplifier. Also positioned at the PM tube base is a limiter so that signals from the anode of the PM tube, prior to the input of the preamplifier, are limited or clipped at 1.2 volts by diodes Dland D2 to reduce spurious signals caused by the overload permitted by prior art limiter circuits. In the prior art, by any such clipping was effected by using diodes beyond the coupling capacitor between the PM tube and the preamplifier.

The preamplifier module is composed of the charge-sensitive amplifier, the first differentiator, and a line-driver output group. These components are essentially conventional. A scintillation within a sodium iodide detector liberates a small burst of electrons from the photocathode of the PM tube, and the burst arrives at the tubes output anode amplified a hundredthousand fold or more. The output is fed by way of a coupling capacitor C5 to the field effect transistor (F ET) input (01) of the charge-sensitive amplifier. This amplifier (Q1, Q2, Q3, O4) is a standard circuit that provides a charge on the 3.9 pf capacitor (C6), which has a discharge time constant of 390 usec. The output of this group serves as the input to the first RC differentiator (R24, C9). This differentiator provides the first pulse shaping of the detected signal, reducing the decay time of the pulse. Resistor R22 provides pole-zero compensation to eliminate undesired undershoot of the differentiated signal. This feature is described in application Ser. No. 504,273, filed Oct. 23, 1965, and having a common assignee with the present application. Transistor stages 05 and Q6 serve as a low-impedance driver for the lOO-ohm cable 8 that connects the detector with the linear amplifier. The input signal to this line-driver group is limited at 1.2 volts by diodes Dd and D5 to prevent very large overloads that may be caused by cosmic rays from reaching the main amplifier. With proper termination, this cable may be of any length.

LlNEAR AMPLIFIER The linear amplifier module 3 shown in FIG. 3a, 3b consists of a shaping amplifier, an inverting amplifier, and the main amplifier. It is linear to an output voltage of 10 volts with a maximum output of 20 volts. The amplifier is DC-coupled throughout except for the differentiating stage; this helps to prevent baseline shifts with changes in count-rate and with overloads. Each stage of the linear amplifier has a band-pass of 10 MHz.

The input to the linear amplifier is a positive-going signal applied at the base of O7. Shaping amplifier O7 through 0110 Model No. Ca3030 IC) has a limited positive signal capability,

the signal is inverted by the inverting amplifier (Q11 through Q15). Coarsegain variations may be made by the multiposition switch SW1, located on the amplifier panel. This coarse control provides a selection in gain from 1.5 percent to 100 percent of full gain. To maintain a balanced input to the operational amplifier, the coarse gain control steps along two balanced resistor strings. The fine gain control, also located on the panel, varies R28 in the feedback loop of the inverting amplifier. its range is from 1 to 0.45, which bridges the 1:2 steps in the coarse gain control.

The operational amplifier is used as the main amplifier group This particular amplifier, using manufacturer-approved compensation networks, is operated with feedback to provide a maximum gain of about 50, with the gain selected as described above. Transistor stages Q16 and Q17 provide a low-impedance output for this group. This is followed by an output amplifier (transistors Q18Q22) which provides a gain of and a low-impedance drive for the lOO-ohm cable 10 connecting the linear amplifier with the single-channel analyzer. A switch SWZA-B provides a selection of operation in either the singleor double-differentiated mode. Under double differentiation, the DC feedback to the base of transistor O8 is taken from the output of transistor Q17. Should a second differentiation not be desired, the DC feedback is taken from the output of the entire linear amplifier.

The feedback circuit is particularly important in a DC-coupled amplifier where the total gain may possibly be as high as 3000. This particular feedback is referred to as an up-down" circuit using a pair of diodes D'O9 and a large capacitor C24. If the potential to the feedback circuit rises, D9 conducts more, tending to cut off D8. The potential on the capacitor rises on a long time constant, and the feedback signal overcomes the output DC level variation.

PULSE-HEIGHT ANALYZER 24V and +24V terminals. The zener diode diode pairs (D10- Dll, D14-D15) at each end further stabilize the voltage I along the string. The firing point for the lower discriminator is determined by the setting of a IO-turn 5K helipot (R12) or E- dial." The window" or AE between the firing of the two discriminators is set by a three-turn ZKhelipot (R5) in the string. Temperature compensations is provided by diodes D12 and D13.

Also contained in this bias string are two spring-retum, pushbutton switches (SW4, SW5) or IS-shift" buttons, one applying a small increase, the other an equal decrease, to the set E-dial value. Since spectral peaks are somewhat rounded, the E-dial setting for maximum count-rate can best be found by equalizing the two offset count-rates produced by the E- shift buttons. A 3 percent shift is enough to move a narrow pass band to the steeply sloping sides of the peak, where inequality is easily detected by means of the auditory count-rate indicator.

' The main functional group of each discriminator is a Darlington-type differential pair, e.g., Q36-Q39. Transistors Q36 and Q37 of the difierential pair are normally cut off and will remain in this state until the signal level at the base of 036 reaches 0 volts, the bias held on the base of Q39. Any input signal equal to or greater than the bias set on the E-dial (10 to 0 volts) will cause Q36 and Q37 to conduct, cutting off 038 and Q39. Capacitor C7 serves as a speedup capacitor for the change of state. The resulting positive-going signal on the col lector of Q38 excites the Schmitt trigger (O42, 043), the output of which is shifted in level and inverted (by O44, O45) for feedback to turn on 041, which changes the reference point of Q39 from 0 volts to the E-dial bias. The differential pair will then return to its original state onlywhen the input signal returns to 0 because of the clamping action of 041. This causes the Schmitt trigger to return to its stable state, completing the formation of the discriminator output pulse. This is differentiated by R59, C20 and the negative-going portion used in the following stages. Diode D24 in the differential pair circuit is a Zener diode used to shift a voltage level.

As stated above, the upper discriminator is identical in construction and its operation is, therefore, the same as that of the lower discriminator. It is triggered whenever the input signal to the analyzer equals or exceeds the sum of the values set on the E-dial and AE-dial. When it is triggered, a negative-going square wave output pulse occurs at the collector of transistor Q34. A switch SW6 is provided at the base of transistor Q46.to permit operation of the analyzer in either a differential mode or an integral mode. In the differential mode of operation, the anticoincidence stage (Q46) prevents the passage of pulses from the lower discriminator to the one-shot multivibrator (048-049) should the input pulse to the unit 4 fire both discriminators. in this case, the output signal of the upper discriminator saturates Q46, shorting the output of the lower discriminator. This circuit ensures that, in the differential mode, only those input signals whose amplitudes fall between the lower and upper reference levels will be counted. In the integral mode of operation, Q46 is disabled and all pulses generated by the lower discriminator are passed to the oneshot multivibrator. The output of this multivibrator is a l zsec, 12 volt pulse which is passed tothe sealer-timer module (described below) through a lOO-ohm cable driven by the low-output-impedance driver stage (QED-Q53).

SCALER-TIMER The sealer-timer module 5 for the spectrometer is shown in the logic and signal fiow diagram of FIGS. 5a, 5b. This module, like the other modules described hereinabove, may be utilized as an entity with other circuitry to record either the number of counts in a preset time period, or the length of time required to accumulate a preset number of counts. This choice is selected by the mode switch (SW7ASW7BSW7C) shown in FIG. 5b.

The two inputs to this module are a 60-cycle timing reference and the output from the aforementioned pulseheight analyzer 4 or equivalent). The timing signal (3600/min.) is shaped and properly inverted and thereafter fed into scaled-down-factor integrated chip elements (S1, S2) that lead to the mode switch SW75. Similarly, the signals from the unit 4 are shaped and inverted and passed to the mode switch SW7A.

The main functional units in this module are two scales: (1) a blind sealer (33-88), which determines the turnoff point of the counting operation, and (2) a readout scaler (SQ-SM), which displays the desired count or time. As mentioned above, the inputs to these sealers are interchanged by the mode switch SW7A, 7B, 7C. In the preset time (PT) mode, the readout scaler counts the unit 4 pulses while the blind sealer counts the power line signals, delivering a turnoff pulse at the end of an interval ranging from 0.1 to 200 minutes as preselected by a switch (SW8). A MAN. position on the switch provides for very long counts with manual shutoff. In the preset count (RC) mode, the readout scaler counts the power line signals, dividing by 36 for hundredths of a minute, while the blind sealer, counting the unit 4 pulses, provides a turnoff pulse at the end of a preselected count ranging from 500 to 40,000 as selected by the scale switch (SW9). In either mode, the turnoff pulse is fed to FPS. This applies a ground signal to the start-stop flip-lop (FFl which shuts off both the input count from the unit 4 and the power line signal by closgate G7. Should the operator prematurely terminate a count by pushing the STOP button SW12, counting can be resumed by pushing the START button SWll. If the count runs to termination by the blind sealer, however, the RESET button SW33 must be pressed before a new count can be started.

In FIGS, 50, 5b, all gates Gl-GS and flip-flops FFI, FF2 are SN7400N: elements SI, S2 are SN7492N; elements S3- S6 and 39-314 are SN7490N; elements S7SS are SN7493N; and elements 1CllC6 are-SN7 441N.

AUDITORY COUNTING RATE INDICATOR The auditory counting rate indicator, referred to as a howler, is shown in FIGS. 6a, 6b. Basically, it utilizes the frequency derived from beating the output of a fixed frequency oscillator against that of an oscillator whose frequency is varied by the counting rate. This beat frequency then drives a conventional speaker or horn.

Referring to FIGS. 6a, 6b pulses from the output of the aforedescribed pulse-height analyzer 4 (same as input to timer-sealer circuit) are fed into a one-shot multivibrator (Q54Q57) for the standardization of the pulses in a one-toone correlation. The width of the multivibrator output pulses is selected by the range switch SW14. These pulses pass through diode D16 into a rate meter circuit (RES, C). The charge on the capacitor C15 is proportional to the number of input counts. The voltage on the base of emitter follower stage Q58, which is proportional to the charge, is therefore also proportional to the incoming counts. This transistor (25% is a buffer stage to keep the RC circuit from being affected by the following circuits.

The voltage output from Q58 is used to modulate one of the two oscillator (Q59) in the howler circuit. The other oscillator (Q64) produces a fixed frequency output signal and this is fed into one side of a mixer (QM-Q63) where it is heat against the variable oscillator frequency. The output frequency from the mixer, in the audio range, drives the speaker through an audio amplifier (Q65-Q70) for producing a sound whose pitch is proportional to the counting rate. A voltage supply circuit 12 provides :12 volts for the auditory counting rate circuit.

l-IlGF-VOLTAGE SUPPLY A completely solid-state high-voltage supply has been provided for the spectrometer. The circuit forthis supply is shown in FIGS. 7a, 75. Primary regulation is accomplished in a rather conventional manner using a series regulator element; in this case a high-voltage transistor (Q80). A voltage doubler 13 supplies approximately 1400 V DC to the collector of this transistor. To achieve regulation, a selector switch SW15- --SW l6 picks off a fraction of the output voltage, and this is compared with the voltage of a temperature-compensated Zener diode (D17) by means of transistors O73, O74, O81. Any voltage difference is amplified by a high-gain X 50,000) differential amplifier (07!, O72, O73, O74, O75, O76, Q81) and applied as a correction signal over lead l4 from base to emitter of the series-regulating transistor Q80. Power for the differential amplifier is supplied by a small power supply 15 giving :12 volts with reference to the highvoltage output terminal. Both this small supply and the differential amplifier float" at the output potential, using the high-voltage terminal as their reference To protect the regulating circuit against extreme variations in line voltage, a preregulator acting on the AC supply voltage has been included in the circuit. Consider that the output of the voltage doubler at V AC input is nominally 1400 V SC. Since the output is proportional to the input, at V AC input the voltage doubler would supply 2180 V DC to the series regulating element. This voltage would exceed the rating of the series transistor Q80, and the power supply would be destroyed.

To prevent this high-voltage condition, a full-wave bridge rectifier (DES-D21) supplying a power transistor (077) is connected in series with the primary winding of the power supply transformer 16 to the AC mains l7, 18. This power transistor Q77, is driven by a small'amplifier (O78, O79) which samples the output from a secondary winding of the power transformer. So long as-this secondary winding supplies less than 10 V the power'transistor Q77 is kept saturated. Any increase in secondary voltage brings this transistor out of saturation; it then drops voltage across its emitter to collector terminals. The total effect of the circuit is to allow only 90 V AC to appear across the power transformer primary; the preregulator circuit absorbs the difference between 90 V AC and the supply voltage. The voltage supplied to the transformer therefore has a truncated sine waveform; this waveform benefits the rectifier-filter circuits in the high-voltage supply since its peak and rms values are more nearly equal than those of a true sine waveform. Transistor 082 provides a starting current for the regulator transistor Q77 when the power is first turned on; once the AC voltage reaches 90 V, Q82 no longer affects circuit operation. Since the preregulator transistor can dissipate a fair amount of power, all the voltage supplies of the spectrometer are supplied via the preregulator'.

As mentioned hereinabove, by the use of all transistors in the various modules described above, it is possible to provide a complete spectrometer unit that is compact and portable. In addition, by employing substantially direct coupling within and between the various modules of the system, there is ensured a substantial improvement in the linearity of the overall system as well as control of noise level and overloads, and by providing means for preregulating the high-voltage power supply, there is ensured a substantially constant output voltage therefrom thus preventing overloading of the power transistor used therein regardless of any extreme variations in input line voltage.

It should be understood that the above-described system was specifically designed for use with scintillation-type detectors. However, the system is not limited to such use and may be used with any type of detectors for the original scanning of radioactivity. It may also be used with data compression systems so as to place scan data on storage tape for subsequent computer processing. Furthermore, the individual major modules may be used separately, if desired, or in various combinations in systems where their functions are desired.

This invention has been described by way of illustration rather than by limitation and it should be apparent that it is equally applicable in fields other than those described.

I claim:

1. In a medical nuclear spectrometer including a photomultiplier tube for receiving light signals from a radiation detector, a measuring count-rate circuit connected to the output of said photomultiplier, said count-rate circuit including a preamplifier, a linear amplifier, a pulse-height analyzer, a

scale-timer, and an auditory count-rate indicator, and a highvoltage supply circuit connected to said photomultiplier and including a power transformer having a primary winding connected to an AC supply voltage input and having a first secondary winding, and means connected to and associated with said first secondarywinding for providing a regulated highvoltage output to said photomultiplier, the improvement wherein said high-voltage supply circuit further includes means for preregulating said AC supply voltage to said supply circuit to thus allow only a predetermined maximum AC voltage to appear at said transformer primary winding, said preregulating means includifiga power transistor, a full-wave bridge rectifier connected in series with said transformer primary winding, said rectifier supplying said power transistor, a second secondary winding coupled to said primary winding of said power transformer, a small amplifier connected to said second secondary winding for sampling the output therefrom, the output of said small amplifier coupled to said power transistor for driving said transistor, whereby so long as said second secondary winding supplies less than a selected voltage, the power transistor is kept saturated, and when said selected voltage is exceeded the transistor is brought out of saturation and then proportionally drops the voltage across its emitter to collector terminals thereby allowing only said predetermined maximum voltage to appear across said transformer primary winding, said preregulating means being thus adapted to effectively absorb any difference between said predetermined maximum voltage and the supply voltage when the supply voltage exceeds said predetermined maximum voltage.

2. The' spectrometer set forth in claim 1, wherein said predetermined maximum voltage is volts, and said selected voltage of said second secondary winding is 10 volts. I

3. The spectrometer set forth in claim 1, wherein a coupling condenser is connected between the output of said photomultiplier and the input to said measuring count-rate circuit, said improvement further including a diode clipper connected.

between the photomultiplier output and the coupling condenser for limiting the output signals from said photomultiplier thereby substantially reducing spurious signals caused by any overloads.

4. The spectrometer set forth in claim 1, wherein said measuring count-rate circuit and saidhigh voltage supply circuit are all transistorized to provide a compact unit, and wherein said improvement further includes substantially complete direct coupling of all circuitry of said count-rate circuit and said high voltage supply circuit, whereby said spectrometer will operate in a substantially linear manner.

UNITED STATES PATENT OFFICE Certificate of Correction Patent No. 3,569,701 March. 9, 1971 Marion M. Satterfield et 111. It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below.

Insert the omitted sheets of drawings containing FIGS. 3b, 4b, 5b, 6b, and 7b:

MENTOR-E Marlon M. SoHeriieId By George R. Dyer W 4. WM

ATTORNH:

OUTPUT mvmdns Q 4 b Marian M. Suflqdield 3y George 8. Dyer AT TORNEY.

in 2nd 310 Mir 5"! 6H! DECADE DECADE DECADE DECADE DECADE DECADE INVENTORS. Marion M Sullarfinld 5b Georg: R. Dyer Attest:

EDWARD M. FLETCHER, Jm, ROBERT GO'ITSCHALK, Oommissiomr of Patents.

Atteating Oficer. 

1. In a medical nuclear spectrometer including a photomultiplier tube for receiving light signals from a radiation detector, a measuring count-rate circuit connected to the output of said photomultiplier, said count-rate circuit including a preamplifier, a linear amplifier, a pulse-height analyzer, a scale-timer, and an auditory count-rate indicator, and a highvoltage supply circuit connected to said photomultiplier and including a power transformer having a primary winding connected to an AC supply voltage input and having a first secondary winding, and means connected to and associated with said first secondary winding for providing a regulated high-voltage output to said photomultiplier, the improvement wherein said highvoltage supply circuit further includes means for preregulating said AC supply voltage to said supply circuit to thus allow only a predetermined maximum AC voltage to appear at said transformer primary winding, said preregulating means including a power transistor, a full-wave bridge rectifier connected in series with said transformer primary winding, said rectifier supplying said power transistor, a second secondary winding coupled to said primary winding of said power transformer, a small amplifier connected to said second secondary winding for sampling the output therefrom, the output of said small amplifier coupled to said power transistor for driving said transistor, whereby so long as said second secondary winding supplies less than a selected voltage, the power transistor is kept saturated, and when said selected voltage is exceeded the transistor is brought out of saturation and then proportionally drops the voltage across its emitter to collector terminals thereby allowing only said predetermined maximum voltage to appear across said transformer primary winding, said preregulating means being thus adapted to effectively absorb any difference between said predetermined maximum voltage and the supply voltage when the supply voltage exceeds said predetermined maximum voltage.
 2. The spectrometer set forth in claim 1, wherein said predetermined maximum voltage is 90 volts, and said selected voltage of said second secondary winding is 10 volts.
 3. The spectrometer set forth in claim 1, wherein a coupling condenser is connected between the output of said photomultiplier and the input to said measuring count-rate circuit, said improvement further including a diode clipper connected between the photomultiplier output and the coupling condenser for limiting the output signals from said photomultiplier thereby substantially reducing spurious signals caused by any overloads.
 4. The spectrometer set forth in claim 1, wherein said measuring count-rate circuit and said high voltage supply circuit are all transistorized to provide a compact unit, and wherein said improvement further includes substantially complete direct coupling of all circuitry of said count-rate circuit and said high voltage supply circuit, whereby said spectrometer will operate in a substantially linear manner. > 